The invention relates to a process for checking the surface of a material, in particular cast aluminum strip, by visual inspection of an image of the material surface provided by at least one camera monitoring the material and supplying the image to a screen via a video signal. The invention also relates to a device for this purpose.
Known so-called on-line quality control methods are carried out for example using magnetic scattering, ultra-sonics, induction currents, or also in a trivial manner by visual inspection of the surface of the material by an observer. Today considerable use is made of television so that the surface is imaged either as a whole or in part and displayed on one or more TV monitors. By freezing the image briefly stationary pictures are obtained of the moving material. By selecting TV cameras of specific spectral sensitivity (e.g. in the infra red range) and by filtering the incident light it is possible to optimize the contrast of the colored picture for ease of recognition of a fault by the observer. The linear scanning of a TV picture permits a computerized record of surface defects by comparison with standards; this can then be processed to register commands for sorting or control for treatment of the surface.
The advantages of the optical methods lie in their applicability to items at any elevated temperature. The optical method enables the avoidance of wear or damage to detectors by the moving item thanks to the large camera-object distance which can be selected. In additon, they have the ability to image the whole or the greater part of the surface of interest instead of the spot or linear inspection possible with all other methods.
On the other hand it is a disadvantage that this surface inspection is possible only at relatively slow strip speeds. At higher speeds only individual parts are displayed as a stationary picture, which then permits only partial inspection e.g. 20% of the total surface area.
The faster the movement of the strip, the poorer and less often are the flaws detected.